1. Field of the Invention
The present invention relates to a loudspeaker apparatus that reproduces sound by applying vibration to an acoustic diaphragm by a magnetostrictive actuator.
2. Description of the Related Art
A loudspeaker apparatus that reproduces sound by applying vibration to an acoustic diaphragm by a magnetostrictive actuator has been suggested.
In particular, Japanese Unexamined Patent Application Publication No. 2007-166027 discloses as shown in, for example, FIG. 8, a configuration in which a cylindrical acoustic diaphragm 110 made of acryl or the like is vertically supported, a plurality of magnetostrictive actuators 130 are arranged at a lower end of the acoustic diaphragm 110, and drive rods 135 of the magnetostrictive actuators 130 contact a lower end surface 112 of the acoustic diaphragm 110, to apply vibration to the acoustic diaphragm 110 in a direction perpendicular to the lower end surface 112, or in a plate-face direction.
In this case, the lower end surface 112 of the acoustic diaphragm 110 is excited with a longitudinal wave. As a vibration elastic wave is propagated in the plate-face direction of the acoustic diaphragm 110, a transversal wave is generated in addition to the longitudinal wave. The transversal wave causes an acoustic wave to be radiated in a direction perpendicular to a plate face of the acoustic diaphragm 110, thereby providing a widely spread sound field.
The magnetostrictive actuator is an actuator using a magnetostrictor the shape of which is changed when an external magnetic field is applied. A giant magnetostrictor is recently available, a deformation thereof being as approximately thousand times large as a deformation of a magnetostrictor in the past. In addition, the magnetostrictor generates a large stress when the shape is changed. Even when the magnetostrictive actuator is small, the magnetostrictive actuator can cause the acoustic diaphragm to produce relatively laud sound, and the magnetostrictive actuator can cause a hard acoustic diaphragm made of, for example, an iron plate to produce sound.
Further, the magnetostrictive actuator has a high response speed. The response speed of a single magnetostrictor is based on a nanosecond order.